Mexico has government vowed to find out whether an explosion that killed 33 people at the headquarters of its state-run oil monopoly Pemex was a deliberate attack or yet another stain on the company’s poor safety record.

Rescue workers are continued to pull bodies from the debris with the search set to continue until officials can account for everyone inside the Mexico City building.

Authorities have refused to speculate over what caused the blast but one survivor believes it was not an attack.

Alfonso Caballero, a Pemex employee injured in the attack said: “Was it an attack? I don’t think so because it was a thing of the moment…there was no explosion, I didn’t hear an explosion, suddenly everything collapsed.”

It is one of the first serious tests for new President Enrique Pena Nieto. He quickly visited some of the 121 people injured and declared three days of national mourning.

A debate over the future of Pemex has raged for some time with many politicians calling for it to be privatised.

The world’s number four oil producer is the biggest contributor to Mexico’s economy but has been plagued by falling production and an abysmal safety record.